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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 7, Misalignment

Section 10, Possible Confusion with Electrical Hum Frequency

Here is a relatively common spectrum that can be misleading. It looks like the harmonic at 4 x rpm is much higher than the harmonics at 2 x and 3 x rpm. However, the apparent peak at 4 x rpm may not be a harmonic at all. It may, instead, be a peak created by an electrically induced vibration.

In the Western Hemisphere, most electricity is generated at 60 cps. This creates a magnetic field that alternates (+ and -) at 2 x 60 cps. If some magnetically affected part is slightly loose (such as iron encapsulated by wires, or slightly loose armature laminations), the loose part will be pushed in one direction at the instant of the positive portion of the electrical cycle and then pushed in the negative direction at the instant of the negative portion of the cycle. The impacts at each end of the cycle cause a hum or buzz at the frequency of 2 x 60 cps = 2 x 3600 cpm = 7200 cpm = 120 Hz. In other parts of the world that use 50 cps current, hum frequency will be at 2 x 50 cps = 2 x 3000 cpm = 6000 cpm = 100 Hz.

With 60 cps current, the most common motor running speed is slightly under 1800 rpm. If for example the actual motor speed is 1780 rpm, the 4th harmonic of 4 x rpm will be 7120 cpm (118.67 Hz). If there is a large enough electrical hum, its frequency will be exactly 7200 cpm, (120 Hz). Within the usual resolution of most vibration instruments, the electrical hum can easily look like the 4th harmonic of running speed. However, if it is actually the 4th harmonic, the symptom can easily be diagnosed as from misalignment or from the vanepass frequency of a 4 vaned pump. If the analyst determines that the frequency is not an exact harmonic, but instead originates from an electrically induced signal, the diagnosis will obviously be entirely different. The same applies for 50 cps current where the most common motor speed is slightly less than 1500 rpm. So when the peak at what appears to be the 4th harmonic is higher than usual, first determine if that peak is a real harmonic or if it is electrically induced.

The same applies even more for motors that are running at or slightly under 3600 rpm (60 cps) or 3000 x rpm (50 cps). What may look like a large 2 x rpm peak can even more easily be confused with a misalignment symptom. For these higher speed motors, the peak will appear to be at or near 2 x rpm rather than 4 x rpm.

Electrical hum frequency can also be mistaken for the 6th harmonic of a 60 Hz electrical motor running approximately 1200 rpm (or a 50 Hz electrical motor at approximately 1000 rpm). The same reasoning can be used for all motor speeds. Although electrical hum frequencies can be confused with misalignment frequencies when near harmonics of 2 x or 4 x rpm, they are also easily confused with vanepass or bladepass frequencies such as at 6 x rpm or higher. (For more detailed instructions, see section "Separating Electrical Hum From A True Harmonic.")

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